A file system in a computer generally is part of the operating system that manages access to one or more storage devices. The file system provides an interface through which other applications can create files, write data to files and read data from files stored on the storage devices. The file system provides an abstraction layer between the details of managing the storage devices, such as the actual storage locations within each device where data is stored, and applications. This abstraction layer allows applications to identify files and access to files using references to files and locations within to files, without the need for the application to have information about the actual storage locations on the storage devices or other details about the storage devices. Typically, the references to files are defined as combinations of various file system objects, such as a name of a storage device or collection of storage devices, names of directories and paths of directories, names of file streams and files, or other types of file system objects. Each file system object generally is a named construct that represents data stored within the file system.
Within the file system, when data for a file is stored, it generally is broken into small chunks (often called blocks) which are stored on the storage device. The file system generally tracks, for each file, the storage locations within the storage device where each block of a file is stored, and the ordering of those blocks.
Some file systems, when writing data in a file, modify the data within the allocated blocks for the file. This is called “in-place” writing. Other file systems, when writing data in a file, are configured to provide a level of data integrity. For example, the file system may compute and store checksums of the stored data. As another example, “copy-on-write” can be used, which involves writing data to a free block of storage in a storage device, then change the file-to-block mapping to refer to the newly written block, with the overwritten data block being discarded. Both checksums and copy-on-write can be used, as well as other techniques for providing data integrity. While providing increased data integrity, techniques such as computing checksums and performing copy-on-write can reduce performance.